I have a very strange error (well to me it is strange anyway).
We have a problem sending email via our domain which is hosted remotely on a dedicated server (which we are responsible for maintaining) but it only happens in specific situations.
To simplify the story, here is the setup:
A laptop roaming and using an internet connections at an-isp.com can send and receive email. It sends email using our dedicated server's SMTP without a problem.
A desktop in the office connected to another-big-isp.com cannot send an email using our server's SMTP, it receives an error like this : "550 5.7.1 <email@example.com>... Relaying denied".
Of course, the desktop can send email using the SMTP servers of another-big-isp.com.
The dedicated server is running Fedora 5, Sendmail and GPL Virtualmin installed as a module in Webmin.
One strange symptom is that the desktop can send and email using our dedicated server SMTP provided it is destined for an email address at another-big-isp.com.
The only common denominator seems to be another-big-isp.com who are telling us that there are no blocked ports or anything else.
Has anybody got any ideas ?
I would greatly appreciate a clue or two.